1. Field of the Invention
The present invention relates to a powered seat slide device, such as a motor-driven seat slide device for an automotive vehicle, and particularly to a powered seat slide device employing at least one pair of guide rails, namely an upper slidable guide rail firmly secured to the bottom surface of the automobile seat and a lower stationary guide rail rigidly mounted on the floor panel of the vehicle body, which is capable of electronically adjusting the position of the automobile seat assembly in a direction generally parallel to the longitudinal direction of the vehicle body.
2. Description of the prior Art
Recently, there have been disclosed and developed various powered seat slide devices which can adjust the relative displacement between upper and lower seat rails. A conventional automotive powered seat slide device traditionally employs a telescopically extendable guide rail structure for each side of the seat assembly. The telescopically extendable guide rail structure is usually interposed between the seat cushion and the floor panel of the vehicle.
Referring now to FIG. 1, there is shown a prior art powered seat slide device for automotive vehicles. The telescopically extendable guide rail structure is comprised of a lower stationary guide rail 60 rigidly mounted on the floor panel of the vehicle body, and an upper guide rail 61 firmly secured to a respective side portion of the base plate of the seat. As is generally known, the upper guide rail 61 is slidably fitted to the lower guide rail 60. The powered seat slide device also includes a guide rail drive mechanism comprised of a gear box 64 including a worm having a driven connection with a drive shaft of a drive motor (not shown) and a worm wheel having external threads meshed with the worm. The gear box 64 serves as a reduction gear for reducing revolutions of the output shaft of the motor and increasing rotation torque created by the motor. As clearly seen in FIG. 1, a screw-threaded shaft 62 is also provided in an aperture defined between the slidably fitted pair of rails 60 and 61. The worm wheel is rigidly formed integral with the screw-threaded shaft 62. The screw-threaded shaft 62 engages with a nut member 63 fixed onto the bottom wall section of the lower stationary guide rail 60 by means of fasteners, such as bolts, so as to prevent both rotational movement and sliding movement of the nut member 63 relative to the lower guide rail 60. Conventionally, the front end (viewing FIG. 1) of the screw-threaded shaft 62 is rotatably supported by bearings provided on the gear box 64, while the rear end of the screw-threaded shaft 62 is rotatably supported by a radial bearing 65 firmly suspended by the upper guide rail 61. In the previously noted conventional powered seat slide devices, since the screw-threaded shaft 62 is rotatably supported at three portions, namely the front end, the substantially center, portion and the rear end, such an arrangement of the screw-threaded shaft 62 requires an excessively precise alignment matching between the bearings of the gear box 64, the nut member 63, and the rear end bearing 65. Supposing that at least one of the above noted bearings and the nut member is offset from a predetermined installation position, there is a tendency for the three supported portions of the screw-threaded shaft 62 to be slightly misaligned with each other. Furthermore, since the upper and lower guide rails 61 and 60 are generally formed by pressing, so as to lighten the entire weight of the guide rail assembly, each guide rail having a relatively low rigidity tends to be slightly deformed by a load applied thereto. Ideally, each supported portion of the screw-threaded shaft 62 must be in alignment with each other. With the previously noted conventional supporting structure of the screw-threaded shaft 62, the respective supported portions of the screw-threaded shaft 62 thus tend to be misaligned with each other, during assembling or after assembling the powered seat slide device on the vehicle body. If the supported portions of the screw-threaded shaft 62 are slightly misaligned with each other, smooth rotation of the screw-threaded shaft 62 is prevented due to undesirable load applied to each supported portion. The screw-threaded shaft 62 vibrates depending on the magnitude of misalignment of the respective supported portions thereof. The greater the misalignment of the respective supported portions of the screw threaded shaft 62 becomes, the greater the amplitude of vibration of the shaft 62. The amplified vibration of the shaft 62 causes undesirable noise. With energy loss resulting from the increased noise, the motor speed fluctuates irregularly. Consequently, the motor employed in the conventional powered seat slide device has an increased tendency to become overloaded due to misalignment between front, middle, and rear supported portions of the screw-threaded shaft 62. This results in a low durability of the device.